Task Manager Disabled

Hi guys ,
I reinstalled XP (fresh) 2 days before.All things were fine.But yesterday evening when I right clicked on the taskbar "Task Manager"option seems to be disabled.So when I pressed Alt. +Ctrl. +Del it displayed warning that Task Manager is disabled by your administrator .I am admin. of my pc

I am using only NOD32 Antivirus and my pc has 24 hrs online connection through lan.Besides NOD32 am not using any adware or firewalls and such kind of softs.

So is it any malicious program or any..........which caused this problem.
Can I enable my task manager by editing registry or any other solution.
Plz reply.